Emplacement of the Santa Rita Flat pluton as a pluton-scale saddle reef

Abstract

Regional mapping indicates that the Middle Jurassic Santa Rita Flat pluton, exposed in the Inyo Range of eastern California, is situated within the core of a south-plunging synform defined by bedding in the surrounding metasedimentary wall rocks, which dip beneath the pluton. However, bedding in rocks preserved above the pluton defines a south-plunging antiform. Anisotropy of magnetic susceptibility analysis revealed that magnetic foliation within the pluton also defines a south-plunging antiform, and the accompanying magnetic lineation plunges subparallel to the axis of this antiform. Our data indicate that the pluton was initially intruded as a sill in the hinge zone of the enveloping synform. Subsequent vertical inflation of the sill resulted in upward doming of the overlying roof and formation of the antiform now observed at the current erosion level in the pluton and preserved roof rocks. Emplacement of the pluton at 164 ± 1.5 Ma (U-Pb zircon age) overlaps in time with regional thrust faulting and folding ca. 185–148 Ma recognized in the southern Inyo Range. We speculate that space for initial emplacement of the pluton was produced during folding by layer-parallel slip and hinge-zone dilation, producing a saddle reef-like structure.